HP-65 - 1605A02841 - Weird display behavior

[teenix]

(02-09-2020 09:04 PM)albertofenini Wrote:  Hi everyone
We are working on this unit but we need your help !
It powers up, but there is a segment always lit no matter what.
Also, the - segment is missing in all the digits so that a 8 is shown as a 0
We thought it was a bad cathode driver but funny thing is that we used a spare keyboard pcb which works fine and it behaves as before
This makes us think that is the logic board faulty
What could we check ?
Pictures can be found here : HP-65 - 1605A02841 weird display behavior
Thanks for help !!!!

Hi Alberto,

The g segment being off all the time might indicate...

a) an anode driver problem
b) an open circuit on the keyboard pcb - anode driver to LEDs
(anode driver pin #20 to LED module pins #1)
c) a faulty inductor for the g segment
d) a faulty ARC in the CPU hybrid module

The c segment being on all the time could be (a) or (d)
This problem indicates the "C" drive line (anode driver pin #1) is logic HI all the time.

Does the problem occur with a different CPU connected?

The only way to see the problem clearer is to look at the 5 partially decoded anode driver lines on an oscilloscope. Looking at the CPU board top, counting from right to left, they are pins 1, 2, 3, 5, 6.

If you are unfamiliar with how the display drivers work, check out my Classic display PDF.

http://www.teenix.org/ClassicDisplay.pdf

cheers

Tony